MOS transistors have been developed in the past that have a semiconductor film with a heterojunction structure in which physical properties of the semiconductor material forming a channel conducting electricity are changed by applying strain to its crystal to improve the mobility in the channel. As shown in FIG. 12 (which is of n-type. Reference numerals of elements of a p-type MOS transistor are enclosed in parentheses.), formed on an Si substrate 101 of such a MOS transistor is a semiconductor film 111 (121) consisting of a SiGe layer 102 (112), which has a compression strain structure, and a Si layer 103 (113), which is a cap film for forming a gate insulating film, stacked in this order. A gate electrode 105 is patterned on a gate insulating film 104 on the semiconductor film 111 (121), and an extension region 106 (116) and source/drain 107 (117) are formed by doping portions of the semiconductor film 111 (121) on both sides of the gate electrode 105 with an impurity. In the MOS transistor, the mobility of positive holes is high because of the compression strain structure of the SiGe layer 102 (112) that is formed on the Si substrate 101 in such a manner that its lattice matches to the Si substrate 101. Thus, especially the characteristics of p-type MIS transistor can be significantly improved.
Japanese Patent Application Laid-Open No. 2002-314089
However, MIS transistors having a semiconductor film with a compressed strain structure have problems described below (FIG. 13). For convenience, the source/drain region 107 (117) shown in FIG. 12 is omitted from FIGS. 13A and 13B.
As shown in FIG. 13A, n-type MOS transistors have problems that:    (1) the mobility of electrons decreases; and    (2) off-current (Ioff) increases because an n-type impurity (for example arsenic (As)) diffuses fast from the extension region 106 into the SiGe layer 102 and the threshold voltage (Vth) shifts significantly in a short channel. Therefore, characteristics of the n-type MIS transistor degrade significantly.
As shown in FIG. 13B, p-type MOS transistor also has problems that:    (1) the mobility of positive holes decreases because of an electric field increasing because an n-type impurity (for example arsenic (As)) fast diffuses in the SiGe layer 112 and therefore the concentration of the n-type impurity in the Si layer 113 increases and the electric field in the Si layer 113 increases; and    (2) the effective mobility decreases because a channel is formed in the Si layer 113 in parallel with a channel in the SiGe layer 112.
In this way, the n-type and p-type MOS transistors having a compression strain structure have their inherent problems, which make it prohibitively difficult to integrate them to give a CMOS transistor having good characteristics.
The present invention has been made in light of these problems and an object of the present invention is to provide a semiconductor device including a semiconductor film having a heterojunction structure with a compression strain structure and a method for manufacturing the same that implements a high reliability by increasing the mobility in the semiconductor device and improving its transistor characteristics.